1. Field of the Invention
The present invention relates to the preparation of polyurethane mixtures, and more particularly the invention is directed to a high-pressure mixing apparatus, of the self-cleaning type, and to a method suitable for the production of polyurethane mixtures with two or more reactive components. Although the method and apparatus according to the present invention are suitable for preparation of any type of polyurethane mixture for making flexible, semi-rigid and rigid foams, according to some aspects disclosed herein, the invention is particularly directed to the preparation of mixtures starting from a total pre-polymer or from polyurethane chemical components having different viscosities.
2. Description of the Related Art
In the preparation of polyurethane mixtures for the production of molded parts, use is made of formulations comprising two or more chemically reactive primary components, suitable for producing a polymer, using a high-pressure mixing apparatus of the self-cleaning type. According to conventional techniques, stoichiometrically metered quantities of the primary components, such as a polyol and an isocyanate, with various additives, are fed separately and injected radially in opposite or angularly spaced positions of a mixing chamber, wherein the various streams frontally collide with one another with sufficiently high kinetic energy to cause strong atomization and total mixing.
As schematically shown in FIGS. 1 and 2 of the accompanying drawings, which represent the general state of the art, in the case in which the mixture is obtained from a polyol and an isocyanate which are premixed with various additives, the two reactant streams are fed into the mixing chamber 13 of a self-cleaning mixing apparatus 10, via diametrically opposed conduits or injection nozzles 11 and 12, between which an angular spacing of approximately 180.degree. exists. This angular spacing is represented by the double arrow 14 in FIG. 2.
Examples of these mixing devices can be found in the literature, from which the following are mentioned by way of example: U.S. Pat. No. 3,706,515, U.S. Pat. No. 3,975,128, U.S. Pat. No. 4,096,585 and U.S. Pat. No. 4,332,335, the complete disclosures of each of which are incorporated herein by reference.
In these apparatuses, during the recycling phases, the necessary seals are provided by narrow dimensions between the external surface of the cleaning member 15 and the internal diameter of the hole defining the mixing chamber, the large angular spacing between the feeding conduits 11 and 12, and the spacing between corresponding recycling grooves normally provided in opposite positions on the same cleaning member. The dimensions can be approximated to within a few microns.
In general, the high viscosity of the primary polyurethane components used in conventional formulations contributes to improving the sealing conditions. Specifically, the high viscosity makes any seepage of one component into the other through the very narrow gap between the inner wall of the mixing chamber 13 and the outer surface of the cleaning member 15 difficult, even during the recycling phase when the same cleaning member is moved fully forwards. Moreover, effective sealing between the primary components during the recycling phases is a critical condition for continued operation of the apparatus. In fact, any contact between the polyol and the isocyanate, or more generally between chemically reactive components during the recycling phases, could lead to the formation and/or deposition of solid particles on the inner surfaces of the mixing apparatus, as well as in the recycling circuits. Such solid particles can damage the apparatus and/or require it to be removed from service for cleaning and repairs.
In an attempt to improve the mixing conditions, as schematically shown in FIG. 3, it has been proposed to slant the feeding conduits 11 and 12 for the polyurethane components to face towards the bottom or a wall of the mixing chamber 13. This technique is shown, for example, in U.S. Pat. No. 3,933,312 and U.S. Pat. No. 4,473,531, the complete disclosures of which are incorporated herein by reference.
However, in this case too, the two primary components are always injected in a substantially radial plane, through conduits or nozzles which open directly in the wall of the mixing chamber 13. Any seepage of one polyurethane component into another, during a recycling phase, is once again prevented not only by a narrow gap between the contacting surfaces of the cleaning member 15 and the mixing chamber 13, but also by a sufficient angular spacing between the conduits 11 and 12.
In other cases, in particular in the preparation of multi-component polyurethane mixtures with a chamber such as shown in FIG. 4, where three or more primary components are fed at high pressure into a mixing chamber 13 through respective conduits 16, 17, 18 and 19, the angular spacing between adjacent conduits is considerably reduced. In FIG. 4, this angular spacing is equal to an angle of 90.degree. or less, if account is also given to the diameters of the feed conduits themselves. In all cases this distance, measured on the contact interface between the mixing chamber 13 and the cleaning member 15, is reduced to about ten millimeters approximately, or less, and is totally insufficient for ensuring the necessary seal, considering that the various polyurethane components are normally recycled at high pressures, on the order of 15-20 MPa (150-200 atm) or higher.
The streams of the various primary components which collide at high speed in the mixing chamber 13 generally have comparatively high flow rates, that is, there is sufficient kinetic energy for ensuring good and complete mixing. Accordingly, in the cases in which the pressure of recycling of the components is considerably high, the angular spacing between the individual conduits and the corresponding recycling grooves is inadequate to ensure sufficient sealing, such as to prevent the seeping of one component into another. Use therefore has to be made of special techniques, such as for example the formation of appropriate longitudinal seals on the cleaning member of the mixing chamber 13, as schematically denoted by reference numeral 20 in FIG. 4.
Examples of longitudinal seals on the cleaning member are represented in DE 2,117,533.
According to this technology, the necessary seals between the feed conduits and the respective recycling grooves for the components are obtained by forming longitudinal slots in the cleaning member of the mixing chamber. The slots are later filled with an appropriate resin, by adopting a method which is somewhat complex and difficult to perform directly in the place of these mixing apparatuses.
Moreover, the resin filling technique for the sealing slots, due to the high wear, has an extremely limited life span, equal to a few working days of the apparatus. Therefore, the resin filling has to be renewed frequently, with subsequent inconvenience and interruptions of the production cycle.
The seal between conduits for feeding the components, between the various recycling grooves respectively, in certain cases is more critical when a total pre-polymer or a polyurethane formulated from TDI is used, i.e., when one of the primary chemical components has a considerably low viscosity or a viscosity that is at least lower than the viscosity of the other component, for example, on the order of a few centipoises.
In general, therefore, the art has always been oriented towards feeding all the primary components in radial directions substantially in the same plane and perpendicular to the longitudinal axis of the mixing chamber through feeding conduits or nozzles which open directly in the side walls of the mixing chamber. Consequently, the sealing during the recycling phase is critical for the reasons already explained.
The so-called "partial pre-polymer" technique has also been well known. According to this technique, only a reduced quantity of an isocyanate is pre-mixed with a polyol, without the cross-linking agent, to obtain a pre-polymer. The pre-polymer thus obtained, and the remaining part of the isocyanate stoichiometrically necessary to complete the polyurethane mixture, are successively injected with additives, such as reaction water, and a cross-linking agent into the mixing chamber of a common mixing device.
Generally, mixing apparatuses of this type have proven to be highly efficacious and suitable to provide good mixing only when the various streams of the components impinging into the mixing chamber have comparatively high yet slightly different flow rates, since all streams must have a sufficiently high kinetic energy to ensure intimate mixing.
Presently, a new technology is being considered which is based on the use of some "total" polyurethane pre-polymers which should be suitably mixed with a metered quantity of a primary reactive additive, for example, water for the production of the CO.sub.2 necessary for foaming, and a suitable agent for the final reticulation or cross-linking.
According to this new technique, a polyol and an isocyanate are pre-mixed in the quantities stoichiometrically necessary to react, to obtain a "total pre-polymer" as defined herebelow. Subsequently, the pre-polymer is mixed with reaction water and a catalyst for the final reticulation. For the purpose of the present invention, the term "total pre-polymer" is used and understood to mean a pre-polymer obtained by pre-mixing stoichiometrically metered quantities of a polyol and an isocyanate with non-reactive additives which are common in the formulations of polyurethane formulations.
This new technology has proven to be efficient, since it allows for the production of polyuretheres with high molecular weights, included in a very limited range not obtainable with the conventional mixing technologies because of the violence and speed of the chemical reaction which occurs among the various chemical components. Therefore according to this total pre-polymer technology, the characteristics of the produced foam are remarkably improved.
Presently, there is a problem in developing and setting up new processes and apparatuses which allow for high-pressure mixing with this new total pre-polymer technology.
Many attempts were carried out to mix a total pre-polymer with a primary additive comprising water and a reticulation catalyst, making use of high-pressure mixing apparatuses of the opposing jet type. However these attempts have produced poor results. In practice, it was not possible to homogeneously mix the primary additive, probably because of its little kinetic energy, since its flow rate constitutes a portion which is in a smaller percentage than the pre-polymer flow rate. Generally, the weight ratio between the reaction water added to the cross-linking catalyst, and the pre-polymer, is indicatively in the range of 10-15% or less. Therefore the catalyst stream has a very low energy, totally insufficient to mix with the pre-polymer. As a consequence, the catalyst is only partially mixed in the pre-polymer and the produced polyurethane foam is defective or for most applications commercially unacceptable.
In the attempt to solve this problem and in the research of a suitable apparatus, tests have been carried out by using the same cleaning member of the mixing chamber as feeding means for the primary additive. The results thus obtained were encouraging and suggested further investigations with this specific testing.
For certain applications it has also been proposed to feed a component axially in the mixing chamber. This is described for example in GB-A-2,036,586, U.S. Pat. No. 4,053,283, U.S. Pat. No. 4,608,233 or in EP-A-594 981, the complete disclosures of which are incorporated herein by reference. In particular GB-A-2,036,586 describes a mixing device for the production of polyisocyanates, or more generally for mixing chemically reactive substances, which uses a special mixing chamber with a bell shape, in which the streams collide in directions substantially at right angles one with another. This mixing device is therefore totally lacking in any mechanical self-cleaning, and the conduits for feeding the various chemical components for mixing are provided directly in the body of the same mixing device. Moreover, a similar mixing device in practice is suitable for continuous mixing processes, for which the control of the stoichiometric ratios between primary components to be mixed is, initially, less critical than discontinuous mixing processes used in molding polyurethane. Therefore, a similar device is wholly inadequate for discontinuous production of polyurethane mixtures, where the stoichiometric control of the mixture components must be strictly ensured right from the start, in theory from the first drop of mixture supplied into a mold.
For this purpose, for some time high-pressure mixing apparatuses of the self-cleaning type, the same cleaning member of the mixing chamber also performs a valve function for simultaneous opening and closing of the injection nozzles, as well as recycling of the various components.
However, in traditional self-cleaning mixing devices, the various nozzles for injecting the components are arranged radially or variously oriented in relation to the longitudinal axis of the mixing chamber, maintaining practically a circumferential arrangement, in the manner discussed previously with reference to FIGS. 1 to 4 and shown in the numerous documents mentioned previously.
Unlike GB-A-2,.036,586, U.S. Pat. No. 4,053,283, U.S. Pat. No. 4,608,233 and EP-A-594 981 show self-cleaning high-pressure mixing devices in which the cleaning member of the mixing chamber is provided with a longitudinal hole which opens axially at the front end in the mixing chamber for feeding a nucleation gas or an auxiliary, not chemically reactive, component.
In particular, in U.S. Pat. No. 4,053,283 a hole for feeding nucleation air extends along the entire cleaning member to lead into a rear air chamber. United States Pat. No. 4,608,223 and EP-A-594 981 also each describes a high-pressure, self-cleaning mixing apparatus, in which the longitudinal hole of the cleaning member is only used for feeding an auxiliary component, such as a dye substance, a releasing agent or anything else for which strict batching of the quantities fed is less critical, or has no effect on the resulting mixture nor on the features of the molded parts. In the aforementioned documents, the polyurethane components, such as a polyol and an isocyanate, are therefore always fed into the mixing chamber by radial jets, in a wholly conventional manner.
From the above, it is clear that, despite the fact that it has been proposed to use an axial hole in the cleaning member of the mixing chamber for feeding a component, the conventional art relating to self-cleaning high-pressure mixing devices has always been oriented towards feeding the primary polyurethane components radially through the facing nozzles which open directly in the wall of the same mixing chamber. Therefore, in cases in which it is necessary to place several injection nozzles closer together, the need arises to use special techniques for the necessary sealing to avoid the risks and disadvantages referred to previously.
Furthermore, contrary to the expectations and experiences associated with conventional high-pressure mixing apparatuses, by testing the total pre-polymer technique it has been found that axially feeding the primary chemically reactive agent into the mixing chamber by a flow crossed with the pre-polymer, and in particular feeding the primary additive axially in the mixing chamber from a slightly rear or back position, very close to the inlet pre-polymer nozzle, at a space equal to or less than the space between the pre-polymer injection nozzle and the outlet of the mixture from the mixing chamber, permits an intimate and homogeneous mixing to be obtained. The stoichiometric ratios of the whole mass of the polyurethane mixture thus formed were respected.
However, during testing, in connection with results generally acceptable, sometime defects were found in the produced foams. The defects were believed to be caused by a non-homogeneous mixing of the cross-linking agent. Upon examination of the problem, a conclusion was reached that the non-homogenous mixing is caused by several factors, including progressive clogging of the feeding hole due to the formation of hard foam scales, difficulty in synchronizing feeding of the various streams at the start of each mixing phase, and interruption of the streams when the system has to pass to a high-pressure re-cycle condition.